Hot-dip galvanized steel sheets used in automobile under body parts are required for further reduction of thickness in order to realize improvement in the fuel cost. For making the thickness reduction of the steel sheet and ensure for parts strength compatible, it has been demanded to increase the tensile strength (TS) of the hot-dip galvanized steel sheet to 1000 MPa or more. Further, in consideration of safety upon collision, it is also demanded to further increase the strength of the steel sheet to a yield strength (YS) of 700 MPa or more. Further, excellent formability is also demanded for the steel sheet in order to be fabricated into underbody parts of complicate configuration. Accordingly, those having a balance of tensile strength (TS) and elongation (total elongation: EL) (hereinafter referred to as “TS×EL balance”) of TS×EL of 24,000 MPa·% or more are demanded.
As a method of improving TS×EL, it has been known to utilize deformation induced martensitic transformation (TRIP effect) of retained austenite (hereinafter also referred to “retained γ”). In the hot-dip galvanized steel sheet, retained γ is generally prepared by an austempering step in a continuous galvannealing line (hereinafter referred to as “CGL”). A matrix is generally classified into those containing and not containing soft ferrite. When the matrix contains ferrite, a steel sheet is excellent in TS×EL but YS is low since ferrite yields preferentially and it was difficult to ensure YS of 700 MPa or more. Ferrite yields preferentially because ferrite is soft in itself and, in addition, mobile dislocations are introduced into the soft ferrite due to expansion of the martensite formed mainly in the final cooling after the alloying treatment during its formation and the mobile dislocations tend to move easily thereby causing plastic deformation at a low stress. On the other hand, when the matrix does not contain the ferrite, there was a problem that a TS×EL balance was poor although YS was excellent. While the TS×EL balance can be improved by making an austempering time longer, the overaging zone for austempering in CGL is generally short and, if the austempering time is increased, this lowers the line speed and deteriorates the productivity. Accordingly, it has been strongly demanded for the development of hot-dip galvanized steel sheets at high productivity (can be manufactured in short austempering time (for example, even within 60 s) and having excellent mechanical properties (hereinafter also referred to simply as properties).
For example, Patent Literature 1 discloses a high strength galvanized steel sheet comprising a bainitic ferrite as a matrix and excellent in the balance of the strength and the workability. However, although the steel sheet achieves TS, YS, and TS×EL at a high level identical with that of the present invention (refer to Experiment No. 28 in Table 4), this involves a problem that the productivity in CGL is poor since this requires an austempering time exceeding 100 s (refer to paragraph [0041]).
On the other hand, Patent Literature 2 discloses an alloyed high strength hot-dip steel sheet excellent in an anti-powdering property. However, while the steel sheet can be manufactured within an austempering time as short as 45 s (refer to paragraph [0124]), some existent steels containing a considerable amount of ferrite satisfy TS×EL of 24000 MPa·% or more, but TS of 1000 MPa or more cannot be ensured (refer to GA steel sheets No. 18 to 23, 25, 36 in Table 5) and it is considered that YS of 700 MPa cannot be ensured. Further, while steels scarcely containing the ferrite may possibly ensure YS of 700 MPa or more, they cannot ensure TS×EL of 24,000 MPa·% or more (refer to GA steel sheets Nos. 27 to 32, 37 to 40, 45, and 46 in Table 5).